1. Field of the Invention
The present invention relates to a thermal-process-type air-flow-rate sensor for measuring an air-flow-rate with a heated resistor. For example, the present invention relates to a thermal-process-type air-flow-rate sensor preferably used for measuring a flow rate of suction air of an internal combustion engine. The present invention also relates to a method of manufacturing the thermal-process-type air-flow-rate sensor.
2. Description of the Related Art
Concerning the air-flow-rate sensor provided in a suction air passage of an internal combustion engine of an automobile, a thermal-process-type air-flow-rate sensor has become the mainstream because a mass flow rate can be directly detected by the sensor. This thermal-process-type air-flow-rate sensor is widely used because the thermal-process-type air-flow-rate sensor having a thin-film-shaped sensor region, which is formed on a semiconductor substrate made of silicon (Si) by applying the semiconductor fine working technique to it, can be relatively easily produced by a mass-production system and, further, this thermal-process-type air-flow-rate sensor can be driven by a little electric power.
Concerning the thermal-process-type air-flow-rate sensor provided with a sensor element having the thin-film-shaped sensor region which is formed on a semiconductor substrate, Japanese Patent Publication No. 9-26343 discloses a floating support structure (a cantilever type supporting structure) in which only one side of the sensor element 10 is bonded to the hollow portion 32 of the sensor support body 30 by the adhesive 48 as shown in FIG. 8A.
In the case where the entire reverse face of the sensor element is bonded to the sensor support body, there is a possibility that the thin-film-shaped sensor region is damaged in the bonding process. Further, according to the condition of using the sensor, there is a high possibility that the bonding portion is damaged by a difference of the coefficient of linear expansion between the sensor element and the sensor support body when the sensor is used being put into a temperature cycle. Accordingly, this type floating type supporting structure is adopted.
In this case, in the thermal-process-type air-flow-rate sensor, when a step is formed between the sensor element surface and the sensor supporting body surface, a state of the air flow on the sensor element surface is changed, which can be a cause of the fluctuation of the characteristic of the thermal-process-type air-flow-rate sensor in the process of mass production in which the adjustment can not be conducted on the individual sensors. In the structure shown in FIG. 8A, as the thickness of the adhesive 48 fluctuates, it is difficult to keep the surface of the sensor element 10 and the surface of the sensor supporting body 30 on the same plane. Therefore, Japanese Patent Publication No. 2001-12986 discloses a structure in which the groove 26 is provided on the reverse face of the sensor element 10 as shown in FIG. 8B and the sensor element 10 is fixed to the sensor supporting body 30 when the groove 26 is filled with the adhesive 48. According to this structure, as the reverse face of the sensor element 10 directly comes into contact with the bottom face of the hollow portion 32 of the sensor supporting body 30, as long as depth D1 of the hollow portion 32 and thickness H1 of the sensor element 10 agree with each other, the surface of the sensor element 10 and the surface of the sensor supporting body 30 can be maintained on the same plane. In this case, in order to secure the detection accuracy required for engine control, it is necessary to maintain a step, which is formed between the sensor element surface and the sensor supporting body surface, in the range not more than 20 to 30 μm.
As the sensor element 10 is subjected to a polishing step, thickness H1 can be maintained substantially constant in the range of allowance of several μm. However, it is difficult to maintain depth D1 of the hollow portion 32 of the sensor supporting body 30 constant. In the case where the sensor supporting body 30 is made of ceramic, it is difficult to maintain depth D1 of the hollow portion 32 constant (in the range not more than the allowance of 20 to 30 μm) in the process of burning, and the yield is deteriorated. Especially when the sensor supporting body is formed out of resin, depth D1 of the hollow portion 32 fluctuates by 100 μm. Therefore, it is impossible to maintain the step in the range not more than 20 to 30 μm.